To answer this question we must look at the way humans generate sounds.
Human voice production can be simply described as a source of slightly
pressurised air (from the lungs) that causes air to flow over vibrating
membranes (the vocal cords) and these vibrations excite the air in a
17-centimetre-long resonance chamber (the trachea plus the mouth and nasal
chambers). By inhaling some helium gas, some of the air in this resonance
chamber is replaced by helium, which increases the propagation speed for
sounds traveling inside the chamber. Ultimately, this process raises the
frequency of resonance sounds emanating from the mouth, as described below.

Because helium is such a light gas it has a sound speed three times faster
than sound in air, so if half
the air in the chamber is helium, the sound speed will be increased by
roughly a factor of 1.5 times the speed in air. The resonance frequencies of
the chamber will correspondingly be increased by the same 1.5 factor, since
the size of the resonance chamber is unchanged and
a resonance frequency is proportional to the sound speed. So it should not
be surprising that the higher resonance frequencies make the human voice
sound `squeaky', or, sound like the cartoon character, Donald Duck. A
listener usually finds that such higher-pitched 'helium speech' is much
harder to understand than 'air speech'.

There is an important application involving helium-affected speech sounds,
as the deep-sea divers regularly use 80 (or even higher) percent helium gas
in the gas mixture that they breathe during long times in the water. It is
important for the divers to be able to converse with other divers and with
people on the surface ships. Some efforts have focused on prior speech
training for the divers. In some applications, electro-mechanical devices
have been constructed so that the diver's communication signal is altered to
one that can be more easily understood by listeners.